An attitude and orbit control system comprises an assortment of on-board hardware and software components that allow a spacecraft to be controlled in order to orient it with the desired attitude and adjust its orbit to the requirements of the mission. In the case, for example, of a geostationary telecommunications satellite, it is sought to keep the orientation of the satellite constant relative to Earth, in order to allow various mission instruments to operate. For this purpose, an AOCS system generally comprises various sensors for detecting the attitude and position of the satellite, actuators, such as propulsion devices and devices for accumulating angular momentum, for modifying its position and attitude, and a flight software package that ensures the orientation and stable orbit of the satellite in the various life phases of the satellite.
During separation of the satellite from its launch vehicle, or when the satellite goes into survival mode following a system failure, the attitude and angular momentum of the satellite are not under control. The initial phase, which consists in stabilising the attitude of the satellite, by reducing its angular momentum, and in orienting it in a desired direction, for example towards the Sun, is a critical phase of the attitude control. The present invention provides a novel method for reducing angular momentum in order to simplify demands on the actuators of the attitude control system and to enable novel satellite architectures, in particular as regards propulsion devices. According to another aspect of the invention, the method advantageously makes it possible to reduce the impact that the angular momentum reducing operation has on the orbit of the satellite.
A common satellite architecture generally consists of a subassembly of hardware and software components, such as telecommunication or observation systems, called the mission subassembly, and what is called a structural subassembly that groups together the hardware and software components required for proper operation of the satellite, from separation with the launch vehicle to the end of the lifetime of the satellite. FIG. 1 shows the structural hardware of a common architecture for an Earth-orbiting satellite. Solar panels 11 are fixed to the structure of the satellite in order to provide the satellite with a power supply throughout its lifetime. An attitude and orbit control system AOCS also forms part of the structural hardware. An AOCS system in particular comprises a set of sensors 12, a propulsion device 13 comprising one or more thrusters 14, and an angular momentum accumulating device 15.
The set of sensors 12, which has the role of detecting the attitude and position of the satellite, for example comprises Earth sensors, Sun sensors and star trackers. Other sensors (gyrometers, accelerometers) allow variations in position or attitude to be measured. The set of sensors 12 delivers an estimation of the position and attitude of the satellite in the three dimensions of space.
The propulsion device 13 in general comprises a number of thrusters 14 fixed in various locations on the structure of the satellite. By delivering a thrust in the direction of the centre of gravity of the satellite, the propulsion device allows the trajectory of the satellite to be corrected and its position on its orbit to be modified. By applying a force away from the centre of gravity, it also allows a torque to be created and therefore the attitude of the satellite to be modified. A first type of thruster, called a chemical thruster, consumes a chemical propellant. It delivers a high-power thrust but has a relatively high consumption meaning that a disadvantageous amount of propellant must be carried on-board the satellite. In a second type of thruster, called a plasma thruster, or electric thruster, xenon atoms are ionised by collision with electrons. Thrust is generated when the charged xenon ions are accelerated out of the thruster by an electromagnetic field. Although they are expensive and have a high initial weight, electric thrusters are substantially more effective than chemical thrusters.
The propulsion device 13 is used both for the transfer from the launch orbit to the mission orbit and to keep the satellite in place on its mission orbit, and to control the attitude of the satellite. To do this, a common satellite architecture generally comprises a number of thrusters allowing the position and attitude of the satellite to be controlled along three axes. To reduce cost and to increase the payload capacity of a satellite, it is desirable to limit the number of thrusters fitted to the satellite and to reduce the amount of fuel required. The use of electric thrusters, which requires a fuel tank that is less heavy and less bulky, is a first optimisation approach. Propulsion systems comprising mechanical means allowing the thrust axis of the thruster to be moved, with the aim of limiting the number of thrusters required, are also known.
An AOCS system also relies on an angular momentum accumulating device 15, such as, for example, a set of reaction wheels, flywheels or gyroscopic actuators. An electric motor drives a flywheel in rotation about an axis of the satellite, a variation in the angular velocity generating a torque that by reaction drives the satellite to rotate about its centre of gravity. An angular momentum accumulating device 15 comprising, for example, three reaction wheels (or four wheels for the sake of redundancy) allows the attitude of the satellite to be stabilised and controlled along the three axes of the satellite.
In practice, operation of the angular momentum accumulating device 15 is closely tied with operation of the propulsion device 13. By applying thrust slightly away from the centre of gravity, it is possible to both modify the trajectory of the satellite and create a torque that can, for example, be used to off-load the reaction wheels while preserving the attitude of the satellite. The software of the AOCS system thus comprises algorithms that centralise the measurements of the sensors 12 and control the position and attitude of the satellite by controlling the propulsion device 13 and the angular momentum accumulating device 15 in the various life phases of the satellite.
FIG. 2 illustrates how a satellite is launched into a geostationary orbit. A launch vehicle transports the satellite to a low-Earth orbit. At the perigee P of the orbit, the satellite is separated and ejected from the payload fairing of the launch vehicle by means of, for example, a mechanical device employing springs. Injection into mission orbit then comprises a number of steps. In a first step, called the SAM (Sun acquisition mode) step, the satellite, the initial angular velocity of which is not controlled, must be stabilised. Its orientation is then kept constant relative to the Sun, the solar panels being partially deployed in order to provide the satellite with a supply of electrical power. When the apogee A of the orbit is approached, the satellite is reoriented in order to allow the propulsion device to deliver a thrust tangential to the orbit. This acceleration near the apogee, which may be repeated over a number of revolutions, allows the eccentricity of the orbit to be reduced until the desired geostationary orbit is obtained. The initial step after separation from the launch vehicle, which consists in stabilising the attitude of the satellite, starting from an uncontrolled initial situation, and achieving a target orientation, is a critical phase controlled by dedicated algorithms of the AOCS system.
This situation is also encountered when the satellite goes into survival mode following a system failure. The same algorithms are then implemented to stabilise the attitude and orient the satellite relative to the Sun until a solution can be found to the system failure.
In known systems deployed at the present time, the propulsion device is controlled in order to generate a torque that opposes the rotation of the satellite. The various thrusters are activated so as to slow rotation in succession on the three axes; this operation in general being repeated a number of times until the angular velocity along each axis is reduced to a value close to zero. After stabilisation, the angular momentum accumulating device is controlled in order to orient the satellite in the desired direction.
However, this approach suffers from drawbacks that the present invention seeks to overcome. In particular, the satellite must be equipped with a large number of thrusters. A common geostationary satellite architecture typically comprises between ten and fifteen thrusters in order to ensure control of attitude and orbit. Electric thrusters, which are heavier and more expensive, cannot be used throughout the propulsion system; chemical thrusters are still necessary, to the detriment of the weight of fuel required.
Reduction in the angular momentum is obtained by the thrust delivered by a thruster. In addition to the torque generated, the thrust causes a movement of the centre of gravity of the satellite, in other words a velocity increment is imparted to the satellite. The velocity increment imparted during the angular momentum reducing phase is another drawback of known systems. In particular, in the case where a number of satellites are launched from the same launch vehicle, the satellites are located very close to each other after separation and any velocity increment represents a collision risk. Likewise, a correction of orbit is necessary after entering into survival mode if the velocity increment is too large.